Implantable cardiac defibrillators (ICD's) are designed to detect cardiac fibrillation and, in response to the detection, to deliver high voltage shock therapy in order to terminate the fibrillation. FIG. 1 is a schematic showing a typical subcutaneous pectoral placement of an ICD 100 in a patient 102, wherein a hermetically sealed and biocompatible canister 104 of ICD 100 houses circuitry to enable detection and therapy delivery via an elongate electrical lead 106, which is coupled to the circuitry and extends distally from canister 104, through the venous system 110 and into the heart 108 of patient 102, for example, the right ventricle RV. Those skilled in the art understand that implantable medical electrical leads, like lead 106, typically include pacing, sensing and defibrillation electrodes. The electrodes of lead 106 are coupled to the ICD circuitry via a connector terminal assembly that terminates elongate insulated conductors of the electrodes, at a proximal end of lead 106; the connector terminal assembly is plugged into a connector module 105, which is mounted on canister 104, to make electrical contact with the contained ICD circuitry via hermetically sealed feedthroughs. Canister 104, for example, formed from a Titanium alloy, is typically employed as a high voltage electrode in conjunction with a high voltage electrode of lead 106 to establish an effective shocking vector for cardiac defibrillation.
Those skilled in the art are familiar with the repetitive stresses of cyclic loading to which implanted medical electrical leads are subjected, and that these stresses make the elongate conductors of implantable medical electrical leads susceptible to fracture after many years of chronic implantation. In certain instances, such as when the wire filars of a lead conductor that completes a sensing circuit for an ICD become fractured, intermittent contact at the fracture site may produce signals that mimic cardiac fibrillation signals and are erroneously detected as such, leading to subsequent delivery of unnecessary high voltage shock therapy. Thus, a variety of conductor configurations and lead body designs have been developed to increase fracture resistance and/or to address such fractures with redundant conductors. Nevertheless, there is still a need for more robust and fail-safe configurations of conductors in implantable medical electrical leads employed by ICD's.